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Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2013
DFA SHORT-TERM GOVERNMENT PORTFOLIO
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading DFA Short-Term Government Portfolio
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the DFA Short-Term Government Portfolio (the “Short-Term Government Portfolio”) is to maximize total returns from the universe of debt obligations of the U.S. Government and U.S. government agencies. Total return is comprised of income and capital appreciation.
Expense [Heading] rr_ExpenseHeading FEES AND EXPENSES OF THE PORTFOLIO
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses you may pay if you buy and hold shares of the Short-Term Government Portfolio.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder Fees (fees paid directly from your investment):
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Short-Term Government Portfolio pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when Portfolio shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Portfolio’s performance. During the most recent fiscal year, the Portfolio’s portfolio turnover rate was 41% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 41.00%
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is meant to help you compare the cost of investing in the Short-Term Government Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock Dimensional Fund Advisors LP (the "Advisor") believes that fixed income investing should involve a long-term view and a systematic focus on bond market risk and return, not on interest rate forecasting or market timing.

In constructing an investment portfolio, the Advisor identifies a broadly diversified universe of eligible securities with precisely defined maturity ranges and credit quality characteristics. The Advisor will then seek to purchase a broad and diverse portfolio of securities meeting these credit quality standards. In making these purchase decisions, if the anticipated maturity risk premium is greater for longer-term securities in the eligible maturity range, the Advisor will focus investment in that longer-term area, otherwise, the portfolio will focus investment in the short-term range of the eligible maturity range. The Advisor also places priority on efficiently managing portfolio turnover and keeping trading costs low.

The Short-Term Government Portfolio seeks to maximize risk-adjusted total returns from a universe of obligations of the U.S. Government and its agencies maturing in five years or less. The credit quality of the securities purchased by the Portfolio will be that of the U.S. Government or its agencies. As a non-fundamental policy, under normal circumstances, the Portfolio will invest at least 80% of its net assets in government securities that mature within five years from the date of settlement. It is the policy of the Portfolio that the maximum length of maturity of investments will not exceed five years from the date of settlement. However, investments may be made in obligations maturing in a shorter time period (from overnight, to up to five years from the date of settlement). The Advisor expects that the Portfolio's average portfolio maturity and average portfolio duration will be three years or less. Duration is a measure of the expected life of a fixed income security that is used to determine the sensitivity of a security's price to changes in interest rates. The longer a security's duration, the more sensitive it will be to interest rates. The Portfolio will also acquire repurchase agreements backed by U.S. government securities. The Portfolio is authorized to invest more than 25% of its total assets in U.S. Treasury bonds, bills and notes and obligations of federal agencies and instrumentalities.
Risk [Heading] rr_RiskHeading PRINCIPAL RISKS
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Market Risk: Even a long-term investment approach cannot guarantee a profit. Economic, political, and issuer-specific events will cause the value of securities, and the Short-Term Government Portfolio that owns them, to rise or fall. Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.

Interest Rate Risk: Fixed income securities are subject to interest rate risk because the prices of fixed income securities tend to move in the opposite direction of interest rates. When interest rates rise, fixed income security prices fall. When interest rates fall, fixed income security prices rise. In general, fixed income securities with longer maturities are more sensitive to these price changes.

Credit Risk: Credit risk is the risk that the issuer of a security may be unable to make interest payments and/or repay principal when due. A downgrade to an issuer's credit rating or a perceived change in an issuer's financial strength may affect a security's value, and thus, impact the Short-Term Government Portfolio's performance. Government agency obligations have different levels of credit support and, therefore, different degrees of credit risk. Securities issued by agencies and instrumentalities of the U.S. Government that are supported by the full faith and credit of the United States, such as the Federal Housing Administration and Ginnie Mae, present little credit risk. Other securities issued by agencies and instrumentalities sponsored by the U.S. Government, that are supported only by the issuer's right to borrow from the U.S. Treasury, subject to certain limitations, and securities issued by agencies and instrumentalities sponsored by the U.S. Government that are sponsored by the credit of the issuing agencies, such as Freddie Mac and Fannie Mae, are subject to a greater degree of credit risk. U.S. government agency securities issued or guaranteed by the credit of the agency may still involve a risk of non-payment of principal and/or interest.

Income Risk: Income risk is the risk that falling interest rates will cause the Short-Term Government Portfolio's income to decline.
Risk Lose Money [Text] rr_RiskLoseMoney Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading PERFORMANCE
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table immediately following illustrate the variability of the Short-Term Government Portfolio’s returns and are meant to provide some indication of the risks of investing in the Portfolio. The bar chart shows the changes in the Portfolio’s performance from year to year. The table illustrates how annualized one year, five year and ten year returns, both before and after taxes, compare with those of a broad measure of market performance. The Portfolio’s past performance (before and after taxes) is not an indication of future results. Updated performance information for the Portfolio can be obtained by visiting www.dimensional.com.

The after-tax returns presented in the table for the Short-Term Government Portfolio are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown in the table. In addition, the after-tax returns shown are not relevant to investors who hold shares of the Portfolio through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table immediately following illustrate the variability of the Short-Term Government Portfolio’s returns and are meant to provide some indication of the risks of investing in the Portfolio. The bar chart shows the changes in the Portfolio’s performance from year to year.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.dimensional.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading DFA Short-Term Government Portfolio Institutional Class Shares
Total Returns (%)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2003-December 2012

Highest Quarter    Lowest Quarter
6.59 (10/08-12/08)    -3.22 (4/04-6/04)
Performance Table Heading rr_PerformanceTableHeading Periods ending December 31, 2012
Annualized Returns (%)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the Short-Term Government Portfolio are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred In addition, the after-tax returns shown are not relevant to investors who hold shares of the Portfolio through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
DFA SHORT-TERM GOVERNMENT PORTFOLIO | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.17%
Other Expenses rr_OtherExpensesOverAssets 0.03%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.20%
1 Year rr_ExpenseExampleYear01 20
3 Years rr_ExpenseExampleYear03 64
5 Years rr_ExpenseExampleYear05 113
10 Years rr_ExpenseExampleYear10 255
2003 rr_AnnualReturn2003 2.69%
2004 rr_AnnualReturn2004 2.74%
2005 rr_AnnualReturn2005 0.77%
2006 rr_AnnualReturn2006 4.51%
2007 rr_AnnualReturn2007 4.95%
2008 rr_AnnualReturn2008 8.36%
2009 rr_AnnualReturn2009 1.46%
2010 rr_AnnualReturn2010 4.45%
2011 rr_AnnualReturn2011 3.39%
2012 rr_AnnualReturn2012 1.59%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 6.59%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Jun. 30, 2004
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (3.22%)
One Year rr_AverageAnnualReturnYear01 1.59%
Five Years rr_AverageAnnualReturnYear05 3.82%
Ten Years rr_AverageAnnualReturnYear10 3.47%
DFA SHORT-TERM GOVERNMENT PORTFOLIO | Return After Taxes on Distributions | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 1.13%
Five Years rr_AverageAnnualReturnYear05 2.91%
Ten Years rr_AverageAnnualReturnYear10 2.29%
DFA SHORT-TERM GOVERNMENT PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 1.20%
Five Years rr_AverageAnnualReturnYear05 2.80%
Ten Years rr_AverageAnnualReturnYear10 2.31%
DFA SHORT-TERM GOVERNMENT PORTFOLIO | BofA Merrill Lynch 1-5 Year US Treasury & Agency Index (reflects no deduction for fees, expenses, or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.98% [1]
Five Years rr_AverageAnnualReturnYear05 3.35% [1]
Ten Years rr_AverageAnnualReturnYear10 3.36% [1]
[1] Source Merrill Lynch, used with permission. MERRILL LYNCH IS LICENSING THE MERRILL LYNCH INDICES "AS IS," MAKES NO WARRANTIES REGARDING SAME, DOES NOT GUARANTEE THE ACCURACY AND OR COMPLETENESS OF THE MERRILL LYNCH INDICES OR ANY DATA INCLUDED THEREIN OR DERIVED THEREFROM, AND ASSUMES NO LIABILITY IN CONNECTION WITH THEIR USE.